Oxygen is typically injected into molten metal baths for such purposes as the refining of steel. For example, steel is refined in electric arc furnaces (EAF) and basic oxygen furnaces (BOF) by injecting oxygen into a melt that contains iron and scrap. The injection of oxygen reacts with carbon, silicon, manganese, iron and impurities that include phosphorus to adjust the carbon content of the melt and to remove the impurities. The oxidation reactions produce a slag layer over the top of the melt. Oxygen is injected for other purposes, for example, copper, lead and zinc for smelting purposes.
It is important that oxygen penetrate the molten metal bath. In the BOF, excessive oxygen reaction in the slag layer causes uncontrolled foaming, leading to the wasteful ejection of material from the converter, a phenomenon called “slopping”. In the EAF, poor oxygen penetration can result in unwanted oxidation of the carbon electrodes, resulting in increased operating costs. Additionally, deep penetration of the oxygen, from a metallurgical lance, will produce a beneficial stirring action of the molten metal.
In order to achieve deep penetration, metallurgical lances have been placed close to the surface of the melt as possible. A problem with this is that the service life of the lance becomes very short due to the intense heat generated at the surface of the molten metal. Another problem is the enhanced risk of releasing water coolant into the furnace, which can result in violent and dangerous reactions with the melt, due to overheating of the lance. Also, deposits form on the molten metal lance that decrease its service life. Another detrimental effect is that molten metal and slag can splash resulting in a loss of product and furnace maintenance problems.
In order to avoid placing the metallurgical lance close to the surface of the melt, it is desirable that the oxygen be discharged from the metallurgical lance with as high a velocity as possible so that the oxygen may penetrate the molten metal while at the same time the lance may be positioned at a distance above the melt. However, when an oxygen jet is discharged from the metallurgical lance, the jet will interact with the furnace atmosphere. Such interaction causes a decay in the velocity and concentration of the oxygen jet and a consequent decrease in the ability of the oxygen jet to penetrate the molten metal bath.
In order to overcome this problem, it has been known to provide a flame envelope or shroud that envelops the oxygen jet to inhibit velocity decay. For example, in U.S. Pat. No. 3,427,151, oxygen is introduced into a nozzle that is provided with a central passageway having a constriction to achieve a sonic velocity at the constriction and a sonic velocity of the oxygen jet being discharged from the nozzle. Supplementary oxygen and fuel is ejected from concentric rings of oxygen passageways and fuel passageways that surround the central passageway to produce the flame envelope that surrounds the central oxygen jet.
U.S. Pat. No. 5,599,375 discloses a burner/injector having a central converging-diverging passageway to inject oxygen into a combustion chamber. Surrounding the converging-diverging passageway are fuel passages to inject fuel in the combustion chamber. Surrounding the fuel passages are secondary oxygen passages to introduce a second oxidizing gas into the combustion chamber. When the burner injector operates in a fuel burning mode, the fuel is combusted within the combustion chamber together with centrally injected oxygen and the second oxidizing gas. This creates a scrap heating and melting flame directed through the combustion chamber towards the scrap to be melted. Once a small portion of the scrap is melted, the flow of the fuel is reduced and the flow of oxygen is increased to create a highly oxidizing flame that rapidly reacts with preheated scrap to melt additional scrap by heat released from the exothermic oxidation. The fuel flow is then further reduced or completely eliminated and the flow of the oxygen discharged from the converging-diverging nozzle is further substantially increased, preferably to a supersonic velocity, to react with an additional portion of the preheated scrap located further away from the burner/injector.
As can be appreciated, U.S. Pat. No. 3,427,151 having only a constricted passageway and not a converging-diverging passageway is incapable of projecting a supersonic jet of oxygen. While U.S. Pat. No. 5,599,375 utilizes a converging-diverging passageway to produce a supersonic jet of oxygen, no flame envelope is employed because little or no fuel is injected and therefore, the supersonic jet of oxygen rapidly will decay due to interaction of the jet with the furnace atmosphere.
In order to solve these problems, U.S. Pat. No. 5,814,125 provides a method of injecting a gas into liquid melt such as molten iron. In accordance with the method, a supersonic jet of oxygen is created within a nozzle having converging-diverging passageway. The supersonic jet of oxygen is surrounded by a flame envelope that is produced by ejecting fuel and oxygen from an inner and outer concentric arrangement of passages surrounding the central converging-diverging passageway. The flame shroud inhibits velocity decay of the supersonic jet of oxygen and allows the oxygen to impact the surface of the liquid melt at distances of 20 nozzle diameters or greater with a supersonic velocity. In U.S. Pat. No. 6,604,937, a gas such as oxygen can be passed through a plurality of outwardly angled converging-diverging nozzles to produce jets having a supersonic velocity for injection into molten metal for refining purposes. Surrounding the converging-diverging nozzles are a ring of ports for alternately ejecting fuel and an oxidant to support combustion of the fuel. Such combustion produces a single flame envelope to surround the jets and thereby to inhibit velocity decay of the jets.
Even when a flame shrouded supersonic jet of oxygen is ejected from an injector or lance, such as described above in U.S. Pat. Nos. 5,814,125 and 6,604,937, molten metal and slag can form deposits known as skull that can clog the openings of passages from which fuel and oxygen are ejected. Such accretion can interfere with the formation of the flame shroud and thereby degrade the utility of the jet or render it ineffective. In order to solve this problem, published Japanese patent application 2002-288,115 discloses a water-cooled lance assembly having a converging-diverging passageway to eject a supersonic jet of oxygen from the lance tip. The supersonic jet of oxygen is surrounded by a flame produced within the central converging-diverging passageway by the internal injection of fuel within the passageway that is combusted within the passageway. In order to stabilize the flame, a straight section of the nozzle that communicates between the end of the diverging section of the passageway and the face of the nozzle is provided with a circumferential groove in which fuel and oxygen collects, decelerates and is combusted upon ignition.
Potential safety and operational problems can arise from the combustion occurring within the nozzle. The combustion of fuel is an exothermic oxidizing reaction that can degrade the nozzle itself to cause eventual or rapid, catastrophic failure. Such degradations can negatively impact lance lifetime and raise the risk of releasing water coolant into the furnace, which can react violently with the melt. There are safety hazards associated with the mixing of the hydrocarbons and oxygen within a confined space in that a combustible, if not explosive, mixture can be created. Those skilled in the art will appreciate the difficulties associated with the requisite ignition, combustion stabilization and flame supervision procedures.
As will be discussed, the present invention provides a method of injecting supersonic jets of oxygen into molten metal is superior to the prior art and in fact minimizes if not eliminates the problems identified in prior art devices discussed above.